1. Field of the Invention
A permanent mold for die-casting light metals, such as aluminum alloy, zinc alloy, magnesium alloy and the like, is associated with a chill vent as means which functions when charging a molten metal into the mold cavity, for efficiently exhausting residual air and/or gas from inside to outside of the cavity, without spouting the molten metal or forming flashing.
The present invention relates to a chill vent used as gas exhausting means upon die-casting such light metals and specifically aims to improve the cooling efficiency of non-solidified molten metal entering into the chill vent to efficiently achieve an accelerated solidification.
2. Description of the Related Art
When air and/or gas is left in the cavity of the permanent mold at the time of die-casting, the air or gas tends to be dragged into the molten metal so as to cause gas holes and the like defects in the products and thereby degrade the product quality.
Therefore, as shown in FIG. 1, it is a conventional practice to provide a permanent mold with a chill vent 3 having a gas exhaust passage 2 which is communicated with the cavity 1 for pressure-casting a product, so that gas remaining in the cavity 1 can be discharged. In FIG. 1, reference numeral 4 designates a die casting permanent mold, and 5 a plunger for forcing out the molten metal.
In this instance, as shown in FIG. 1, the gas exhaust passage 2 is generally shaped in a zigzag-manner to ensure that, after the gas has been exhausted outside the chill vent, the molten metal is chilled in the passage 2 before it is flashed outside the permanent mold.
However, since the molten metal flows under a high-pressure condition, it is difficult to completely prevent the flashing of the molten metal even if the length of the passage 2 is increased by the zigzag shape.
In order to prevent flashing of molten metal with an improved reliability, it was considered necessary for the zigzag-shaped gas exhaust passage 2 to have a narrow gap d, or adopt a relatively steep angle .theta. of the zigzag-shape (waveform).
However, a narrow gap d causes the sectional area of the gas exhaust passage 2 to be decreased, while a steep angle .theta. causes the gas exhaust resistance to be increased. In any case, the gas exhaust efficiency is lowered and it becomes impossible to prevent formation of gas hole defects in the product.
Further, when the length L of the chill vent is increased, flashing of the molten metal can be prevented without narrowing the gap d of the zigzag-shaped gas exhaust passage 2 or adopting a steep angle .theta. of the zigzag-shape. However, such a measure results in increased size of the chill vent and difficulties for meeting with recent requirement for small-sized devices.
There have been proposed various types of chill vents which are capable of efficiently exhausting internal residual gas and preventing flashing of the molten metal, without increasing the size of the chill vent 3.
However, these proposals are still accompanied by problems that the structure becomes complicated and/or large-scaled auxiliary devices are required.
That is, in the former case, with reference to the basic structure such as that shown in FIG. 2, an elaborated arrangement is required such as a composite structure of telescopic elements 6 for the chill vent 3, which causes the entire chill vent to be complicated in shape and/or structure.
Further, in the latter case, with reference to a representative arrangement such as that shown in FIG. 3, it is necessary to arrange a gas suction device 7 adjacent to the chill vent 3 in order to further improve the gas exhaust efficiency. In this instance, although the size of the chill vent itself is not increased, the entire facility including the auxiliary devices is necessarily increased in size and troublesome and costly to manufacture.
In order to solve the above-mentioned problems, a proposal was made in Japanese Patent Application No. 9-57,572 wherein a chill vent is formed of a copper alloy having a superior thermal conductivity, and wherein a cooling pipe is provided adjacent to a zigzag-shaped gas exhaust passage (FIG. 4).
In this instance, it is possible to realize an improved cooling property of non-solidified molten metals entering into the chill vent, to thereby effectively prevent flashing of the molten metal without complicating the structure or increasing the size of the chill vent, with the size and shape maintained unchanged as before.
However, use of copper or copper alloy as the material for chill vent resulted in a new problem as explained below.
That is to say, in order to allow assembly of the permanent mold, the parting surfaces are designed such that the parting surface of the chill vent is 1/100 to 5/100 mm higher than the parting surface of the cavity mold.
Furthermore, when the permanent mold is assembled, a fastening force of typically several tons to 2,500 tons is applied depending upon the scale of die casting machine.
Conventionally, even when assembly of permanent mold is performed under the above-mentioned conditions, there had been raised no particular problem since both cavity mold and chill vent were made of SKD61 or the like having a high coefficient: of elasticity. When, however, a chill vent is made of copper or copper alloy having a low coefficient of elasticity, the chill vent is subjected to a plastic deformation by the applied fastening force. On the other hand, the cavity mold is applied with the fastening force subsequently to the chill vent and undergoes an elastic deformation since it is made of a material having a high coefficient of elasticity.
As a result, after die casting has been completed and the fastening force removed, only the parting surface of the chill vent is slightly depressed as compared to its peripheral portions, and this may cause leakage or flashing of molten metal.